Electrocatalytic metal hydride generation using CPET mediators

Abstract

Transition metal hydrides (M-H) are ubiquitous intermediates in a wide range of enzymatic processes and catalytic reactions, playing a central role in H⁺/H₂ interconversion¹, the reduction of CO₂ to formic acid (HCOOH)² and in hydrogenation reactions. The facile formation of M-H is a critical challenge to address to further improve the energy efficiency of these reactions. Specifically, the easy electrochemical generation of M-H using mild proton sources is key to enable high selectivity versus competitive CO and H₂ formation in the CO₂ electroreduction to HCOOH, the highest value-added CO₂ reduction product³. Here we introduce a strategy for electrocatalytic M-H generation using concerted proton-electron transfer (CPET) mediators. As a proof of principle, the combination of a series of CPET mediators with the CO₂ electroreduction catalyst [Mn^I(bpy)(CO)₃Br] (bpy = 2,2'-bipyridine) was investigated, probing the reversal of the product selectivity from CO to HCOOH to evaluate the efficiency of the manganese hydride (Mn-H) generation step. We demonstrate the formation of the Mn-H species by in situ spectroscopic techniques and determine the thermodynamic boundary conditions for this mechanism to occur. A synthetic iron-sulfur cluster is identified as the best CPET mediator for the system, enabling the preparation of a benchmark catalytic system for HCOOH generation.